Interpretive Summary: Glyphosate-resistant (GR) soybean is the most successful transgenic crop in the world that provided formers the flexibility to manage weeds. However, application of glyphosate to GR soybean results in significant injury under certain conditions. Scientists at the Southern Weed Science Research Unit, Stoneville, MS and Natural Products Utilization Research Unit, University, MS hypothesized that, if GR soybean is completely resistant to glyphosate, then injury could be caused by a metabolite of glyphosate, aminomethylphosphonic acid (AMPA), a known phytotoxin. Greenhouse studies evaluated glyphosate and AMPA effects on GR and non-GR soybean. Application of AMPA at 0.25 kg/ha produced injury in both GR and non-GR soybean similar to that caused by glyphosate at 13.4 kg/ha in GR soybean. AMPA levels found in AMPA-treated soybean of both types and in glyphosate-treated GR soybean correlated similarly with phytotoxicity. These results suggest that injury in GR soybean was caused by AMPA formed from glyphosate degradation.

Technical Abstract:
Glyphosate-resistant (GR) soybean was developed by stable integration of a foreign gene that codes insensitive enzyme 5-enolpyruvylshikimate-3-phosphate synthase, an enzyme in the shikimate pathway, the target pathway of glyphosate. Application of glyphosate to GR soybean results in significant injury under certain conditions. We hypothesized that if GR soybean is completely resistant to the glyphosate molecule, injury could be caused by a metabolite of glyphosate, aminomethylphosphonic acid (AMPA), a known phytotoxin. We examined glyphosate and AMPA effects on one- to two-trifoliolate leaf stage (16- to 18-d old) GR and non-GR soybean in the greenhouse. In GR soybean, single applications of glyphosate-isopropylammonium (1.12 to 13.44 kg/ha) with 0.5% Tween 20 reduced chlorophyll content of the second trifoliolate leaf by 0 to 10% at 7 d after treatment (DAT) and shoot dry weight by 0 to 8% at 14 DAT compared with Tween 20 alone. A single application of AMPA (0.12 to 8.0 kg/ha) with 0.5% Tween 20 reduced chlorophyll content of the second trifoliolate leaf by 0 to 52% at 4 DAT and shoot fresh weight by 0 to 42% at 14 DAT in both GR and non-GR soybean compared with Tween 20 alone. AMPA at 0.25 kg/ha produced injury in both GR and non-GR soybean similar to that caused by glyphosate-isopropylammonium at 13.44 kg/ha in GR soybean. AMPA levels found in AMPA-treated soybean of both types and in glyphosate-treated GR soybean correlated similarly with phytotoxicity. These results suggest that soybean injury to GR soybean from glyphosate is due to AMPA formed from glyphosate degradation.